Method and apparatus for testing a vehicle charge storage system

ABSTRACT

Method and apparatus for accurately measuring charge storage system operating conditions in a vehicle. A measurement circuit produces an electrical response at an output which varies linearly with voltage levels measured via a remote connection to the battery terminals. The circuit is calibrated to provide that electrical levels are produced at the output only over a voltage range above a specified voltage level at the battery terminals in order to improve the resolution in detecting these levels using a standard electrical level detector such an ammeter or voltmeter. The measurement circuit is connected to a display circuit for indicating at least one charge storage system operating condition based on the terminal voltage of the battery. Alternatively, the measurement circuit may be connected to a display circuit for indicating the specific gravity of the battery based on the terminal voltage of the battery.

FIELD OF THE INVENTION

This invention relates generally to vehicle charge storage systems. More particularly, the present invention concerns a method and apparatus for accurately measuring the operating characteristics of components in a vehicle charge storage system and for electronically measuring the electrolyte specific gravity of a battery.

BACKGROUND OF THE INVENTION

The operating condition of a charge storage system which is included in a vehicle, such as, for example, an automobile, truck, aircraft or powerboat, may be determined by evaluating the battery percent charge level, the battery health and/or the charging system health. Alternatively, the electrolyte specific gravity of a battery may be measured for purposes of determining the charge level.

The electrolyte specific gravity of an unsealed battery is typically determined by performing measurements directly at the cells of the battery using a conventional hydrometer float. Such measurements are very time consuming and also potentially hazardous due to the highly volatile gases that often escape from the battery cells.

The typical display on a hydrometer provides a specific gravity reading that is accurate to approximately 10 percent. Further, before a specific gravity value can be interpreted for determining operating conditions in a charge storage system, the specific gravity value on the display may need to be adjusted to compensate for variations in temperature. As a result, for safety and accuracy reasons, a conventional hydrometer float is not generally used for testing the charge storage system in a vehicle.

It is most common that testing of a vehicle charge storage system is performed by measuring the potential difference at the terminals of a battery in a vehicle. This measurement is typically performed when the battery is in an open circuit condition. The open circuit terminal voltage (OCTV) of a battery may be converted into a battery percent charge level, which provides an indication of battery operating condition. See, detailed discussion below.

Charge storage system testing devices and, specifically battery testing devices, which perform OCTV measurements often require periodic recalibration and typically do not include electronics and displays for providing highly accurate readings of the battery percent charge level for expected values. The battery testing industry, for example, uses battery testing devices which include a programmed microprocessor unit that models the internal resistance and capacitance of the battery from OCTV measurements for purposes of providing an indication of battery internal resistance and charge level. These testing devices, however, are typically too costly or too large in size to allow for practical use by the public.

A need, therefore, exists for an economical, convenient, safe and accurate method and apparatus for determining the charge level and health of vehicle charging system components.

SUMMARY OF THE INVENTION

In accordance with the present invention, a test meter for testing a charge storage system in a vehicle is provided. The test meter includes a measurement circuit for measuring the open circuit and loaded terminal voltage of the battery and a display connected to the output of the measurement circuit for displaying indicators of the operating condition of the charge storage system-based upon open circuit and loaded terminal voltages. The output of the measurement circuit responds only when the terminal voltage of the battery exceeds a threshold level, for instance 11 volts d.c., and varies in linear relation to all terminal voltages above this threshold value. Thus, a full meter scale display of voltages, for instance, between 11 and 14 volts d.c., is therefore made possible. In another aspect of the present invention the same measurement circuit is connected to a display circuit for indicating the specific gravity of the battery based on the terminal voltage of the battery.

Further features and advantages of the present invention will become readily apparent to those of ordinary skill in the art by reference to the following detailed description and accompanying drawings.

DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an exemplary Scale Expander Circuit for testing a vehicle charge storage system in accordance with the present invention.

FIG. 2 illustrates a test meter which includes the circuit of FIG. 1 and three scales for providing an indication of the operating condition of a vehicle charge storage system in accordance with the present invention.

FIG. 3 illustrates a test meter which includes an alternate exemplary Scale Expander Circuit and provides an indication of the specific gravity of a battery in accordance with the present invention.

FIG. 4 illustrates an alternate exemplary Scale Expander Circuit for testing a vehicle charge storage system in accordance with the present invention.

FIG. 5 illustrates a specific embodiment of the test meter of FIG. 3.

DETAILED DESCRIPTION

FIG. 1 shows an exemplary embodiment of a Scale Expander Circuit (SEC) 5 which may be suitably used for detecting the potential difference at the terminals of a battery for testing the operating condition of components in a vehicle charge storage system. Advantageously, this circuit expands aselected segment at the top end of a voltage source so that the selected segment may be displayed over the entire scale of an analog meter or on a digital display. Thus, expected values for the tested voltage may be more accurately measured and displayed.

Referring to FIG. 1, the SEC 5 may suitably include a voltage expander path15 which comprises a series connection of a potentiometer 21 and two zener diodes 22 and 24. The potentiometer 21 of the path 15 is an input to the SEC 5, and is suitably connected to the terminals of a battery in a vehicle as explained in detail below. The anode of the zener diode 24 is connected to an output A of the SEC 5. As more fully discussed below, the output A may be suitably connected to any type of electronic circuit block(e.g., analog meter, LEDs or digital display) which provides indications ofthe operating condition of components in a vehicle charge storage system based on measurements performed at the terminals of a battery using the SEC 5.

In one embodiment of the present invention, the potentiometer 21 may be suitably connected across the terminals of a battery in a vehicle charge storage system via a remote connection. This remote connection to a battery provides for safe monitoring of charge storage system operating conditions using a simple, accurate and compact charge storage system monitoring device in accordance with the present invention. For example, the SEC 5 may be suitably connected to any convenient location in a vehicle, such as an accessory fuse terminal in the fuse panel of the vehicle, which permits measurement of the potential difference at the battery terminals.

In the preferred embodiment, the potentiometer 21 is connected to a battery38 that is included in a vehicle charge storage system 40 of a vehicle 50 via a standard cigarette lighter socket 30, which is also included in the vehicle 50. The lighter socket 30 connects the input of the SEC 5 across the terminals of the battery 38 through a cigarette lighter fuse 32. The vehicle charge storage system 40 further includes an alternator block 39 which is coupled to the terminals of the battery 38 using standard techniques. For ease of reference, the potential difference across the terminals of the battery 38 is hereinafter referred to as V_(b).

The SEC 5 detects a range of voltages V_(b) that may appear at the terminals of the battery 38. In accordance with the present invention, thezener diodes 22 and 24 are operated above the reverse breakdown voltage (10.4 volts d.c.) to provide electrical levels at the output A which vary in linear relationship with the voltage levels V_(b) above 11 volts d.c.which may appear at the battery 38 and be detected at the input to the SEC 5. Operation where V_(b) is below 11 volts d.c. will not produce the linear circuit response required for obtaining an accurate measurement of the range of V_(b) values that may be detected between full charge and discharge.

The zener diodes 22 and 24 and the potentiometer 21 in the path 15 operate in combination as a voltage switch that remains closed until V_(b) is greater than 11 volts d.c. The switch opens at this point and the display is monitored for purposes of testing charge storage system operating conditions, for example, between 11 and 14 volts. The zener diodes 22 and 24 of the preferred embodiment are standard 5.2 volt Motorola 1N4689 typesthat act as a series switch having a 10.4 volt drop across it. The potentiometer 21 is suitably adjusted to provide that a voltage will appear at the output A, or a current level may be produced from the outputA, only if V_(b), measured as the OCTV or loaded terminal voltage at the battery 38, exceeds a predetermined level. Further, the potentiometer 21 provides that a range of resistive loads may be inserted in the path 15 such that standard voltage or current detection devices may be connected to the output A of the SEC 5 for purposes of accurately testing the operating conditions of components in the vehicle charge storage system 40.

The SEC 5, thus, is elegant in its simplicity for providing the unique and novel advantage of operating as a voltage divider type of switch that linearly reduces the range of changes in potential difference at the battery terminals for measurement using a suitable detector and prevents the development of an electrical level at the output until a specific voltage is measured at the battery terminals. For example, the SEC 5 may be compensated by adjusting the value of the potentiometer 21 to accommodate the scale factor of a current detecting device, such as an ammeter, which is connected to the output A that will not begin to measurea current produced at the output A until a voltage equal to or greater thanthat of a fully charged battery is detected at the input to the SEC 5. In another aspect of the invention, described in greater detail below, the connection of a suitable voltage detecting means, such as an analog-to-digital converter or voltmeter, to the output A of the SEC 5 permits determination of the electrolytic specific gravity of a battery with high accuracy.

FIG. 2 illustrates a test meter (TM) 100 which includes the SEC 5 and may suitably provide indications of the following vehicle charge storage system operating conditions: (1) charging system health; (2) battery percent charge level (BPCL); and, (3) battery health. In accordance with the present invention, the TM 100 comprises the SEC 5 which is suitably connected to an analog display for performing remote measurement of the potential difference at a battery for purposes of accurately determining and displaying charge storage system operating conditions. For purposes ofhighlighting the advantages of the present invention, an explanation of theabove-mentioned vehicle charge storage system operating conditions is initially provided. Then, the structure and operation of the TM 100, whichprovides for suitable connection of the SEC 5 to the battery 38, is explained in detail.

The health of a vehicle charging system is a general representation of the operating condition of the alternator block 39. The alternator block 39 may typically include devices such as an alternator belt, an output diode bridge and a voltage regulator which are connected to each other and ultimately the battery 38 in a well known manner. The alternator block 39 provides for charging of the battery 38 when the engine, not shown, of thevehicle 50 is running or idling. According to the present invention, the TM100 may be utilized for indicating the health of the vehicle charging system based on measurements of V_(b) which are performed using the SEC 5 when the engine of the vehicle 50 is running or idling, in other words, when the alternator block 39 should be charging the battery 38. It is wellknown that an alternator block that is operating properly, called a healthycharging system, will typically maintain the potential difference at the battery terminals between 13.6 and 14.8 volts d.c. as a function of ambient temperature.

The battery percent charge level (BPCL) is a well known indicator of the open terminal charge level of a battery. BPCL may be determined by measuring the potential difference at the battery terminals with no load applied and when the battery is not being charged. According to well established mathematical relationships, it can be shown that BPCL is directly proportional and varies linearly with open circuit terminal voltage (OCTV) at a battery. For a battery in satisfactory operating condition, an OCTV level of 11.89 volts d.c. corresponds to a fully discharged battery having a BPCL equal to 0%, and an OCTV level of 12.68 volts d.c. corresponds to a fully charged battery having a BPCL equal to 100%.

It should be noted that after charging has ceased, a topping voltage is retained in the electrolyte of a healthy battery for at least several hours or more, unless removed with an appropriate discharge current level.In moderate climates, for example, OCTV will remain at approximately 13.3 volts for at least ten hours after charging has ceased and, therefore, BPCL will remain near or slightly above 100% for this interval. It may, therefore, be desirable to remove this topping voltage using the method described below before measuring BPCL.

Battery health is a term common to the battery testing industry, and refersto the determination of battery internal resistance by measuring the open circuit potential difference across the battery terminals before and aftera predetermined load is applied. A good battery must hold its topping voltage when the load is applied. A bad battery will not. It is well knownthat the topping voltage of a battery which is in marginal operating condition will dissipate almost immediately after charging ceases. For example, BPCL may decrease to between 70 and 90% after charging ceases, thereby suggesting that the battery may not be in satisfactory operating condition. In this case, battery operating condition may be determined by applying a selected accessory load to the battery in a vehicle before attempting to recharge the battery, i.e., before starting the engine. For example, before the engine is started, a head lamp or defog switch in a vehicle may be turned on for approximately one to two minutes. The terminal voltage of a healthy battery will remain at a level higher than apre-determined threshold level when such a load is applied.

This pre-determined threshold level is related to the expected internal d.c. source resistance of a healthy battery. For a healthy battery, the source resistance is typically 0.03 ohms at full charge and 0.283 ohms at full discharge. However, for a malfunctioning or aged battery, the source resistance of the battery typically would exceed the above values, causingthe terminal voltage at the battery to decrease below the predetermined threshold level when a load is applied. A weak battery, for example, may have a source impedance equal to 0.06 ohms at full charge. Thus, the TM 100 provides for measurement of the source resistance of a battery when a load is applied for purposes of indicating battery health based on measurement of the potential difference at the battery terminals.

In accordance with the present invention, the TM 100 including the SEC 5 may suitably measure the potential difference at a battery for providing an indication of vehicle charge storage system operating conditions. In the preferred embodiment, V_(b) is measured using the SEC 5 for accurately determining and displaying charge system health, battery percent charge level and battery health.

Referring to FIG. 2, the TM 100 may suitably comprise a diode 125 and a needle display circuit 140 connected in series to the output A of the SEC 5. The needle display circuit 140 comprises an ammeter winding resistance R, 130 connected to a deflecting needle 150. The deflecting needle 150 moves transversely across a display 160. The display 160 may suitably include scales having indicia for indicating vehicle charge storage systemoperating conditions (e.g., charge system health, battery percent charge level and/or battery health) based upon measurements of V_(b) that are performed using the SEC 5 in accordance with the present invention.

The needle display circuit 140, including the ammeter winding resistance R_(w) 130 and the needle 150 are standard components which are suitably connected for detecting current level through a circuit path, namely the path including the SEC 5 and ammeter winding resistance R_(w) 130, and also for facilitating display of these measured levels on the display 160.For example, a 0 to 1 mA ammeter manufactured by Prime Instruments providesa suitable needle display circuit 140.

The diode 125, which connects the SEC 5 to the needle display circuit 140, provides temperature compensation for counteracting variations in zener break down voltage levels which may occur at the zener diodes 22 and 24 when V_(b) measurements are performed at various temperatures. In the preferred embodiment, diode 125 is a standard signal diode 1N4001 or equivalent. It is to be understood that one or more diodes which are similar in structure and function to the diode 125 may be added in series with the diode 125 to provide for additional forward voltage drops, as maybe required for coupling a highly sensitive current detection device to theSEC 5. Likewise, one or more resistors may also be added in series with thediode 125 for the same purpose.

Referring to FIG. 2, the display 160 may suitably include a charging systemhealth scale 170, a battery percent charge level (BPCL) scale 180 and a battery health scale 190. In a preferred embodiment, the potentiometer 21 is suitably adjusted for calibrating the TM 100 based on the current levels which should be produced at the output A from OCTV measurements of a healthy battery.

Referring to FIG. 2, the scale 180 may suitably include indicia indicating BPCL between 0 to 100 percent. The scale 180 is preferably subdivided intothree colored regions which, from left to right on the display 160, includea red region for 0-25% BPCL, a yellow region for 25-75% BPCL and a green region for 75-100% BPCL. The potentiometer 21 of the SEC 5 is suitably adjusted to produce current levels through the SEC 5 and the ammeter winding resistance 130 which cause the needle 150 to deflect linearly across the range of the scale 180 and indicate BPCLs between 0 and 100% that vary linearly in relation to detected levels of V_(b), or OCTV values between 11.89 and 12.68 volts. As a result, the needle 150 would deflect through the red region of scale 180 for values of V_(b) between 11.89 and 12.09 volts, through the yellow region for values between 12.09 and 11.48 volts and through the green region for values above 12.48 volts.

For purposes of illustration, the potentiometer 21 may be selected as a 1 kohm device and the needle display circuit 140 may operate as a 0 to 1 mA full scale ammeter having an associated winding resistance R_(w) preferably equal to 930 ohms. It has been experimentally shown that BPCL readings obtained using the TM 100 which contains these specific components are accurate to within 5% of the expected value.

The scale 170 is calibrated with reference to the scale 180 and in accordance with the well known principle that the terminal voltage of a battery undergoing alternator charge in moderate climates will vary between 13.6 and 14.8 volts. In the preferred embodiment shown in FIG. 2, the scale 170 includes written indicia of POOR in a red colored region which corresponds to deflection of the needle 150 for V_(b) values measured between 12.28 and 12.5 volts, written indicia of FAIR in a yellowcolored region corresponding to deflection of the needle 150 for V_(b) values measured between 12.5 and 13 volts and written indicia of GOOD in agreen colored region corresponding to deflection of the needle 150 for V_(b) values measured between 13 and 14 volts.

The scale 190 is also calibrated with reference to the scale 180 and suitably includes indicia representative of battery health which are basedon measurement of V_(b) reduction when a load is applied to the battery 38. Scale 190 is based upon differences in needle movement after applying a predetermined accessory load. The test is only performed on a battery having a charge level above 75% (i.e., V_(b) =12.48 volts) on scale 180.Needle movement must be less than 10% of scale 190 from its position prior to load application. Larger movement indicates the need for a service station check. The scale 190 includes written indicia of POOR in a red colored region below 11.7 volts, written indicia of FAIR in a yellow colored region between 11.7 and 12.3 volts and written indicia of GOOD in a green colored region between 12.3 and 14 volts.

It is noted that the scales 180 and 190 utilize different measurements for providing information concerning the operating condition of a battery. Thescale 180 utilizes OCTV measurements to indicate BPCL, whereas the scale 190 utilizes terminal voltage levels obtained when a load is applied to the battery. For example, on the BPCL scale 180, a V_(b) measurement of 12.0 volts corresponds to an indication of a partially discharged battery,whereas, on the scale 190, a V_(b) measurement of 12.0 volts is representative of the internal source resistance of the battery, which is an indicator of battery health.

In one embodiment of the TM 100 an announcer, not shown, may be included aspart of the needle display circuit 140 for providing an audible indication,such as a beeping sound, when an OCTV measurement is made and the needle 150, deflects into the yellow region or BPCL is between 25-75%. This audible indication would inform a user that the battery operating condition is suspect.

The TM 100 may be used in the following manner for testing battery operating condition using the scale 180. For example, when an automobile is operated only on short distance trips, charging of the battery may not last for a sufficient time to allow for full charge of the battery. In this case, measurement of the battery using the TM 100 may typically result in the needle 150 deflecting to the yellow region of the scale 180.This lower BPCL reading does not necessarily indicate the battery is not healthy, rather, the battery may merely need to be fully charged to ensurethat sufficient charge is available at the battery for starting the engine at a later time.

The scale 180 further may be utilized for providing BPCL readings after thetopping voltage is removed from the battery terminals. After charging ceases, the topping voltage may either dissipate naturally or be removed manually or automatically using an automatic discharge circuit which applies at least a 10 ampere discharge current load to the battery 38. In an alternative embodiment, the TM 100 may be modified to include a toppingvoltage removal circuit which includes either an actuating solid state switch or relay that automatically places and maintains a load (e.g., vehicle defogger or headlights) on the battery terminals until the toppingvoltage is suitably removed, that is V_(b) is reduced to 12.68 volts. At this point, BPCL can be accurately determined for providing an indication of battery condition.

In addition, the scale 180 may be used for determining whether some cells of the battery are shorted and, therefore, cannot maintain a charge. When this condition exists, after charging ceases, the OCTV at the battery typically will not be maintained at a level above 12.46 volts. Therefore, BPCL will always read 75% or lower on the scale 180 immediately after charging ceases, as the OCTV will quickly drop to a lower than expected level. If this occurs, further testing using the scale 190 is necessary, as explained below.

The scale 190 provides for measurement of battery health based on the well known fact that the source resistance of a healthy, fully charged battery is typically 0.03 ohms. As described above, battery health may be measuredby manually or automatically applying a selected accessory load, such as a head lamp or defog current or depressing the brake pedal to operate the brake lights, to the battery for approximately one to two minutes after charging ceases. A healthy battery which is under load will maintain its terminal voltage at a level higher than a predetermined voltage level. Theleft-most portion of the yellow region of the scale 190 corresponds to the pre-determined threshold level. Deflection of the needle 150 to the left of this level indicates a battery that is not healthy. For example, application of a load to an aging or partially charged battery would probably result in a lower V_(b) reading than expected, thereby causing less current to be produced at the output A such that the needle 150 defects only into the red region of the scale 190. This reading indicates that the battery may not be healthy. The user, therefore, should attempt to recharge the battery by, for example, driving the vehicle for a substantial amount of time, and then repeat the battery health test.

In an alternative embodiment, the SEC 5 may be used in conjunction with a standard digital display device, LED driver and associated compensation resistor which are connected to the output A in place of the diode 125, the ammeter load 130, needle display circuit 140 and the display 160 for providing an indication of, for example, battery health and battery percent charge level using red and green colored LEDs. For example, the potentiometer 21 may be adjusted with respect to the value of the compensation resistor for illuminating a green LED corresponding to a 100%BPCL reading, which is based on an OCTV measurement of 12.68 volts.

In another aspect of the present invention, the SEC 5 may be utilized for determining the specific gravity of a battery. It is well known that the OCTV of a battery may be measured for purposes of determining electrolyticspecific gravity of a battery as OCTV and specific gravity levels vary linearly in relation to each other. This relationship, which is based on Nernst's Equation, may be expressed as follows: ##EQU1##where: OCTV=Open Circuit Terminal Voltage

R=Rydberg Gas Constant

T=Temperature (OK)

F=Kinetic Activation Factor

a_(H2SO4) =Fraction Molar Concentration of Sulfuric Acid

a_(H2O) =Fraction Molar Concentration of Water

Substituting the appropriate constant values, Equation 1 may be reduced to: ##EQU2##In turn, Equation 2 may be rewritten as:

    OCTV=11.76+0.0043Tln(6.9(s.g.-0.965))                      (3)

where s.g. is the specific gravity of the battery and typically varies between 1.100 and 1.265. This relationship has been verified empirically through many measurements by the industry. Thus, according to most established standards, for a healthy battery, OCTV equal to 12.68 volts corresponds to a specific gravity of 1.268 at 80° F.

FIG. 3 shows an embodiment of a Digital Hydrometer (DHM) 200 including the SEC 5 and driver circuit 208 for providing digital readings of the electrolytic specific gravity of a battery on a digital display 202. The driver circuit 208 in combination with the SEC 5 permits that a specific gravity reading may be performed on sealed batteries without the need for performing measurements directly at the cells of a battery, as the required measurements of the battery may be performed safely and accurately through remote measurement of the voltage at the battery terminals using the cigarette lighter socket 30 or like devices. A conventional hydrometer float, by contrast, is generally not calibrated toprovide electrolytic specific gravity readings above 1.400, which corresponds to an OCTV level of 13.2 volts. Referring to FIG. 3 the DHM 200 suitably comprises SEC 5 connected in parallel to a driver circuit 208and a load resistance 206. Driver circuit 208, in turn is connected to a four digit display 202.

One specific embodiment of the DHM 200, depicted in FIG. 5, includes diode 210 and amplifier 212 connected in series to the input of SEC 5. Amplifier212 is connected as a voltage follower with its input tied to B+ to act as a current source thereby widening the measurement range of the DHM 200. Amplifier 212 (as well as amplifiers 240, 242 and 252) is preferably an op90 and diode 210 is a 1N4002 type. SEC 5 is identical to that described above in connection with TM 100 and load resistance 206 comprises 1N9148 diodes 214, 216, 218 and 220 connected in series.

As described above, output A from SEC 5 is input into driver circuit 208, which in this embodiment is designed to drive a Graylex Industries Model 30 four digit display (not shown). In addition, driver circuit 208 of FIG.5 includes circuitry 256 to calibrate the digital display to accurately display the correct specific gravity based upon the measured OCTV. Switch 244 switches between the output of SEC 5 (i.e., from point A) and the output of calibrate circuit 256, each signal being routed through buffer amplifiers 242 and 240, respectively. The signal selected by switch 244 isrouted through a voltage divider comprising a 2.2 kohm resistor 246, a 250 ohm potentiometer 248 and a 8.2 kohm resistor 250, to amplifier 252. The output from amplifier 252 drives the digital display.

In operation, the MC7805 voltage regulator 222, provides +5 volts d.c. to the Graylex display and to an ADM663 voltage regulator 224 of the calibrate circuit 256. Voltage regulator 224, in turn, is configured with resistors 226 and 228 being 180kohms and 100 kohms, respectively, to provide an output of 4 volts d.c. The 4 volt signal is then run through a voltage divider comprised of 10 kohm resistor 230 and 10 kohm potentiometer 238 to provide a voltage which corresponds to the voltage level that exists at point A when the OCTV of a battery under test is 12.68 volts. In order to calibrate the circuit, switch 244 is switched to the calibrate position to select the output of calibrate circuit 256 through buffer amplifier 240. Potentiometer 238 is then adjusted so that the display reading is 1.265, which would be the approximate specific gravity of a fully charged battery (i.e., V_(b) =12.68 volts) at 80° F. Switch 244 may then be returned to the run position to select the output of SEC 5 and specific gravity measurements of a battery may be taken.

In one alternative of the present invention, the TM 100 and the DHM 200 maybe manufactured as a stand alone, portable charge storage system testing devices. In another alternative embodiment, the DHM 200 and the TM 100 maybe suitably included in a single portable device, where the respective voltage and current detection and display components may be independently connected to the output A of the SEC 5 for providing either a specific gravity reading or charge system operating condition indications.

FIG. 4 shows a Scale Expander Circuit (SEC) 300 which may be used in place of the SEC 5 in either the TM 100 or the DHM 200 for accurately measuring the potential difference at the terminals of a battery. The circuit 300 includes a potentiometer 321 which at a first end is suitably connected toa battery in the same manner as the potentiometer 21 of the SEC 5. The potentiometer 321 at a second end is connected to the collector of a PNP transistor 328. Further, the potentiometer 321 at the second end is connected in series with a resistor 326, which is connected to the base ofthe transistor 328, and in series with a resistor 320 which is connected tothe base of a NPN transistor 330. A thermistor 322 and a resistor 324 connect the base of the transistor 330 to the ground return to the battery. The emitter of the transistor 330 is also connected to the groundreturn to the battery. The emitter of the transistor 328 is connected to anoutput B of the circuit 300.

In accordance with the present invention, the circuit 300 is operated over the breakdown region of the base-emitter diodes of the transistors 328 and330 for obtaining a linear response at the output B for changes in V_(b) which are measured at the input to the circuit 300. The thermistor 322 compensates for reduced current gain at a transistor at lower temperaturesand increased current gain at a transistor at higher temperatures, as well as the decreasing battery voltage and increasing battery internal resistance with decreasing temperature. In other words, well known transistor operating characteristics are exploited in accordance with the present invention to provide that the current produced at the output A varies linearly in relation to V_(b). The components in the circuit 300 are suitably selected for forward biasing the base of transistor 330 when V_(b) is approximately 11.0 volts. Forward biasing the base of the transistor 330 causes the base-emitter diode of the transistor 330 to conduct current. This current, in turn, causes the base voltage of the transistor 328 to reduce sufficiently to cause forward biasing of the base-collector diode of the transistor 328. When this occurs, the emitter of the transistor 328 begins to conduct a current to the output B, which may be suitably connected to an ammeter in accordance with the present invention.

In the preferred embodiment transistors 328 and 330 are type 2N2904 and 2N2222, respectively, resistor 326 is 5 kohms, resistor 320 is 10 kohms, resistor 324 is 608 ohms and thermistor 322 is 2.7 kohms at 77° F. By adjusting potentiometer 321 to approximately 1.3 kohms, this circuit has been found to provide suitable output for an ammeter having a winding resistance of 930 ohms. In addition, SEC 300 produces the desired output characteristics at a significantly reduced cost as compared to the circuitof SEC 5.

The circuit 300 may be similarly connected in place of SEC 5 in the TM 100 as described above for the SEC 5, except that the diode 125 would not be included. Similarly, the circuit 300 may directly replace the SEC 5 in theDHM 200. For example, for the TM 100 including the circuit 300, the deflection needle 150 may be calibrated on the display 160 by adjusting the potentiometer 321 and selecting values for the resistors 324 and 320 of the voltage divider which provide a very small current response at the base of the transistor 330 in response to large voltage changes at the battery 38.

It is to be understood that the embodiments and variations shown and described above are illustrative of the principles of this invention only and that various modifications may be implemented by those skilled in the art without departing from the scope and spirit of the invention. 

I claim:
 1. An apparatus for testing a charge storage system in a vehicle, said apparatus comprising:a measurement circuit for measuring a terminal voltage of a battery in the vehicle, said circuit providing an output which varies in linear relation to the terminal voltage of the battery within an expected range of voltages, wherein said output is provided only when the terminal voltage of the battery exceeds a threshold level; a display circuit for indicating at least one charge storage system operating condition based on the terminal voltage of the battery, said display circuit being electrically connected to the output of the measurement circuit; and means for electrically connecting said measurement circuit to the battery.
 2. The apparatus of claim 1, wherein said connection means comprises means for connecting said measurement circuit to a cigarette lighter socket of the vehicle, wherein said lighter socket is electrically connected to the battery.
 3. The apparatus of claim 1, wherein said connection means comprises means for connecting said measurement circuit to a fused accessory load of the vehicle, wherein said fused accessory load is electrically connected to the battery.
 4. The apparatus of claim 1, wherein the display circuit comprises an ammeter for indicating said at least one charge storage system operating condition on at least one scale.
 5. The apparatus of claim 4, wherein said ammeter comprises a deflecting needle, wherein the deflecting needle indicates said at least one charge storage system operating condition on said at least one scale.
 6. The apparatus of claim 4, wherein said ammeter comprises a light emitting diode array, wherein the light emitting diode array indicates said at least one charge storage system operating condition on said at least one scale.
 7. The apparatus of claim 4, wherein said at least one charge storage system operating condition comprises a battery percent charge level measurement and said at least one scale comprises a battery percent charge level scale.
 8. The apparatus of claim 4, wherein said at least one charge storage system operating condition comprises a charging system health measurement and said at least one scale comprises a charging system health scale.
 9. The apparatus of claim 4, wherein said at least one charge storage system operating condition comprises a battery health measurement and said at least one scale comprises a battery health scale.
 10. The apparatus of claim 4, wherein said at least one charge storage system operating condition comprises three charge storage system operating conditions, a battery percent charge level measurement, a charging system health measurement and a battery health measurement, and wherein said at least one scale comprises three scales, a battery percent charge level scale, a charging system health scale and a battery health scale.
 11. The apparatus of claim 1, wherein said measurement circuit comprises a series path including a first and a second zener diode.
 12. The apparatus of claim 11, wherein said first zener diode has a first break down voltage which varies with temperature, and wherein said second zener diode has a second break down voltage which varies with temperature, and wherein said apparatus further comprises means to compensate for variations in said first and second break down voltages.
 13. The apparatus of claim 12, wherein said compensation means comprises a diode connected in series with said first and second zener diodes.
 14. The apparatus of claim 1, wherein said measurement circuit comprises a first and a second transistor, wherein the first transistor includes a first base, said first base being electrically connected to a voltage divider circuit, and wherein the second transistor includes a second base, said second base being electrically connected to a collector of said first transistor.
 15. The apparatus of claim 14, wherein said first transistor has a first current gain which varies with temperature, and wherein said second transistor has a second current gain which varies with temperature, and wherein said voltage divider circuit comprises means to compensate for variations in said first and second current gains.
 16. The apparatus of claim 15, wherein said compensation means comprises a thermistor.
 17. An apparatus for testing a specific gravity of a battery in a vehicle, said apparatus comprising:a measurement circuit for measuring a terminal voltage of a battery in the vehicle, said circuit providing an output which varies in linear relation to the terminal voltage of the battery within an expected range of voltages, wherein said output is provided only when the terminal voltage of the battery exceeds a threshold level; a display circuit for indicating the specific gravity of the battery based on the terminal voltage of the battery, said display circuit being electrically connected to the output of the measurement circuit; and means for electrically connecting said measurement circuit to the battery.
 18. The apparatus of claim 17, wherein the display circuit comprises:a digital display; and a driver circuit electrically connecting said digital display.
 19. The apparatus of claim 17, wherein said connection means comprises means for connecting said measurement circuit to a fused accessory load of the vehicle, wherein said fused accessory load is electrically connected to the battery.
 20. The apparatus of claim 17, wherein said connection means comprises means for connecting said measurement circuit to a cigarette lighter socket of the vehicle, wherein said lighter socket is electrically connected to the battery.
 21. The apparatus of claim 17, wherein said measurement circuit comprises a series path including a first and a second zener diode.
 22. The apparatus of claim 17, wherein said measurement circuit comprises a first and a second transistor, wherein the first transistor includes a first base, said first base being electrically connected to a voltage divider circuit, and wherein the second transistor includes a second base, said second base being electrically connected to a collector of said first transistor.
 23. The apparatus of claim 17, further comprising means to calibrate said display circuit.
 24. The apparatus of claim 23, wherein said calibration means comprises:a voltage regulator; and a voltage divider circuit.
 25. A method for testing a charge storage system in a vehicle, said method comprising:electrically connecting a test meter to a battery in the vehicle, wherein said test meter includes a display having at least one scale for displaying at least one charge storage system operating condition, and a measuring circuit which provides an output that varies in linear relation to a terminal voltage of the battery within an expected range of voltages, wherein said output is provided only when the terminal voltage of the battery exceeds a threshold level; measuring the terminal voltage of the battery using said test meter; determining at least one charge storage system operating condition based on the terminal voltage of the battery; and displaying said at least one charge storage system operating condition on said at least one scale. 